Semiconductor laser devices such as ridge waveguide lasers and laser amplifiers are used in many communications systems. Incremental refinements in their fabrication and packaging have resulted in a class of devices that have acceptable performance characteristics and a well-understood long-term behavior. Moreover, weakly guiding ridge waveguide structures are less complex to fabricate and provide excellent yields as compared to more complex architectures based on buried heterostructures, for example.
In most applications, maximizing the laser""s or amplifier""s useful operating power is a primary design criteria. In signal laser applications, the power output from the device dictates the distance to the next repeater stage, and the number of stages in a given link is a major cost factor in the link""s initial cost and subsequent maintenance. In pump laser applications, where typically multiple pump laser devices are used to optically pump a gain or amplifying fiber, such as a rare-earth doped fiber amplifier or regular fiber in a Raman pumping scheme, useful power output dictates the number of pumps required to reach a required pumping level and/or the distance between pump/fiber amplification stages.
Under current technology, the typical application for pump lasers is fiber amplification systems that utilize rare-earth doped fiber as the gain fiber. These gain fibers are located at attenuation-dictated distances along the fiber link. They typically are comprised of erbium-doped fiber amplifiers (EDFA). The laser pumps typically operate at 980 nanometers (nm) or 1480 nm, which correspond to the location of absorption peaks for the EDFA""s in the optical spectrum.
More recently, Raman pumping schemes have been proposed. The advantage is that special, periodic, EDFA amplifier gain fiber is not required to be spliced into the fiber link. Instead, regular fiber can be used. The result is a gain spectrum that is even wider than systems relying on EDFA""s. The bandwidth typically extends over the entire transmission bandwidth for fiber, stretching from 1250 to 1650 nm for some fiber compositions. The pump lasers are designed to operate in the wavelength range of 1060 to 1500 nm in the typical implementation.
Advantages associated with Raman amplification systems surrounds the fact that there is no longer a 3 dB noise penalty associated with each amplifier, as occurs with EDFA""S. Raman amplification, however, is a non-linear process. As a result, relatively high pump powers are required.
In any case, high power pumps are required, regardless of whether EDFA""s or regular/Raman systems are used. Currently, pumps yielding 180 to 200 milliWatts (mW) of power are available. Newer system designs are requiring even higher power pumps, however.
As higher pump powers are required, additional optimizations are required in the pump laser module. One subject for these optimizations concerns the laser pump chip within the module and how light generated in this module is coupled through a fiber pigtail to the gain fiber. Specifically, in the present invention, the ridge width of the laser chip is optimized and this optimization is propagated through the module design to provide higher useful power outputs from the laser module.
Specifically, the present invention concerns a ridge waveguide laser module in which the ridge width is greater than 4 micrometers (xcexcm). This is a relatively wide ridge for this class of laser chips. Specifically, especially for signal lasers, narrow ridge widths of 1-3 xcexcm are used in order to control lateral and transverse modes and obtain a relatively round beam for easy coupling into fiber. In the present invention, however, where a chip is used for high power pump applications, a wider ridge is used to decrease the loss associated with the laser mode as well as maximize the effective pump gain volume to thus maximize the useful power and light that is coupled into the fiber pigtail, which is typically single mode fiber.
The wider ridge results in higher power density at the laser exit facet. This is not a problem, however, because facet failure is typically not the failure mode at this wavelength. Thus, wider ridge can be used, which maximizes the power in the lowest order modes.
In general, according to one aspect, the invention concerns a ridge waveguide pump laser module that is adapted to generate light in the 1,200 to 1,600 nm (1.2-1.6 xcexcm) wavelength range. The module comprises a ridge waveguide laser chip having a ridge width greater than 4 xcexcm. An optical fiber pigtail is also provided. An end of this pigtail is positioned to receive the light generated by the laser chip and transmit that generated light, typically to a fiber amplification system.
In specific embodiments, the ridge width is less than 11 xcexcm at a narrowest region of the ridge. This specification applies regardless of the overall ridge profile. Specifically, the wide ridge can be used where the ridge further widens in the direction of the rear facet or in the direction of the front facet of the chip. Additionally, the invention also applies where the ridge is tapered in the direction of the rear and/or front facets of the laser chip.
In the preferred embodiment, the ridge width is between 6 and 9 xcexcm. Presently, 7 xcexcm is believed to be optimum.
In other aspects of the implementation, because a relatively wide ridge is used, the cross section of the laser light emission from the chip is very elliptical, rather than circular as is found in narrow ridge designs. Special coupling techniques are thus preferably used. Specifically, if discrete lenses are used, a cylindrical or cross-cylindrical lens are preferred between the laser chip and the fiber pigtail to focus the emission from the chip onto the end of the pigtail. Alternatively, or in combination with a discrete lens system, fiber lenses are also useful. Specifically, in one embodiment, a flat-top wedge shaped fiber lens is used to capture the elliptical emission. Wedge shaped lenses, however, can also be implemented, along with double-wedge shaped fiber lenses.
In still other implementations, special-core fiber pigtails improve coupling efficiency of the elliptical beam. Specifically, a fiber pigtail with an elliptical cross-section core is useful. Further, the core is preferably flared in the direction of the end that is positioned to receive the light from the laser chip, such that the core has a larger cross-section in that direction. Further, elliptical cone shaped fiber lenses can also be used.
In general, according to another aspect, the invention also features a ridge waveguide pump laser chip. Specifically, the laser chip is adapted to generate light in the 1.2 to 1.6 xcexcm wavelength range. Further, this chip has a ridge width that is greater than 4 xcexcm.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.